We moved recently to Mexico. There is a frozen fish here with a Mexican hechsher. the fish is filleted and has no scales that can be seen. I called the Rabbi in charge of this particular hashgocha because the fish comes from china and he said they send a mashgiach there to watch things while packaging the fish etc. But can we rely upon that? is it okay to use that frozen fish?
As a general rule, a single witness is believed with regard to all matters of issurim. If the rabbi you contacted is God-fearing, and you have no reason to doubt his testimony, you may rely on his hashgachah and use the fish. This is all the more true with regard to the issue of fish, which does not include many halachic stringencies (such as those in slaughtering), but only in ensuring that the fish is a kosher type.
Sources: Rambam (Shechitah 10:14; Eidus 11:7); Shulchan Aruch (YD 127:3) (concerning a single witness who is believed with reagrd to issurim), among many sources